There is a strong demand for smaller, lighter, and longer-life electronic devices such as mobile phones and portable information terminal devices, which have become pervasive over the last years. In this connection, batteries, particularly secondary batteries, that are small and light and capable of providing high energy density have been developed as power supplies. Aside from applications to these electronic devices, the use of the secondary batteries has been investigated in a variety of other areas, including electric power tools such as an electric drill, electric vehicles such as an electric car, and power storage systems such as a home power server.
Among a wide range of secondary batteries that operate under a variety of charge and discharge principles, lithium ion secondary batteries that provide battery capacity by taking advantage of the storage and release of lithium ions are very promising for their ability to provide higher energy density than other batteries such as lead batteries and nickel cadmium batteries.
Lithium ion secondary batteries include a positive electrode, a negative electrode, and an electrolytic solution. The positive electrode and the negative electrode contain active material capable of storing and releasing lithium ions. Typically, Li complex oxides are used as the active material of the positive electrode (positive electrode active material). Typical examples of Li complex oxides include compounds having a laminar rock salt structure (such as LiCoO2), compounds having a spinel structure (such as LiMn2O4), and compounds having an olivine structure such as LiFePO4. Typical examples of the active material of the negative electrode (negative electrode active material) include carbon materials such as graphite, metallic materials such as Si and Sn, and Li complex oxides such as Li4Ti5O12. The active materials are appropriately selected according to such factors as the intended use of the lithium ion secondary battery.
These active materials have been studied in a variety of ways with regard to their compositions and configurations, because the types of active material greatly influence battery performance such as battery capacity and cycle characteristics. Specifically, because the storage and release of lithium ions tend to be slower when using a Li complex oxide of an olivine structure as the positive electrode active material than when using a Li complex oxide of a laminar rock salt structure, there has been a number of proposals directed to improving the storage and release of lithium ions.
In order to improve the charge and discharge capacity for the charge and discharge of large current, supporting conductive fine particles on the powder surface of lithium iron phosphate material is proposed (for example, see Patent Document 1). The lithium iron phosphate material is represented by LizFe1−yXyPO4 (X is Mg or the like, 0≦y≦0.3, 0≦z≦1). The conductive fine particles have a higher redox potential than the lithium iron phosphate material.
In order to obtain excellent input and output densities independent of the charged state, combining lithium transition metal complex oxide particles and carbon substance fine particles is proposed (for example, see Patent Document 2). The lithium transition metal complex oxide is represented by LiMePO4 (Me is one or more divalent transition metals).
In order to obtain excellent electrical conductivity, combining LiFe complex phosphate and carbon material and providing a specific surface area of 10.3 m2/g or more for the complex are proposed (for example, see Patent Document 3). The LiFe complex phosphate is represented by LixFePO4 (0<x≦1), and the complex primary particle size is 3.1 μm or less.
In order to obtain high discharge capacity, by using the spray-pyrolysis technique, producing a lithium composite metal phosphate having a crystallite size of 35 nm or less is proposed (for example, see Patent Document 4). The lithium composite metal phosphate is represented by LixAyPO4 (A is Fe or the like, 0<x<2, 0<y≦1).
In order to obtain high capacity even under current load conditions, producing lithium iron phosphate particles by using a lithium raw material, a phosphorus raw material, and an iron raw material (for example, iron oxide with an average primary particle size of 5 nm to 300 nm) is proposed (for example, see Patent Document 5). The producing method includes mixing the raw materials, adjusting the agglomerate particle size of the mixture to 0.3 μm to 5 μm, and calcining the agglomerate particles under, for example, a reducing gas atmosphere.